As a conventional grid-connected power supply system, there is provided, for example, a grid-connected solar power generation system having a electricity storage device that systematically interconnects a solar power generation apparatus and a commercial power grid and can be charged by both of them. The electricity storage device is charged with power supplied from the commercial power grid in the nighttime when the electricity rate is relatively low and the power discharged from the electricity storage device is supplied to one or more loads when the amount of power to be generated by the solar power generation apparatus is insufficient in the daytime when the electricity rate is relatively high (see Japanese Patent Application Publication No. H10-201129 or Japanese Patent Application Publication No. 2002-369406). The conventional system supplies power, charged in the electricity storage device, to the loads when the amount of power generated by the solar power generation apparatus is small and does not fulfill the power demand of the loads, as in cloudy or rainy weather, thereby suppressing costs (electricity costs) that are incurred by the power supply from the commercial power grid.
However, the conventional electricity storage device converts the AC power, supplied from the commercial power grid, into DC power and then stores it, and needs to reversely convert the DC power into AC power when supplying power to the loads, so that there is a power loss corresponding to the sum of power loss during AC-DC conversion and power loss during DC-AC conversion. For this reason, there may be cases where in fine weather in which the solar power generation apparatus can generate sufficient power, power (energy) savings and reduction in electricity costs can be achieved by supplying power directly from the commercial power grid, rather than compensating for the deficit with the power that was charged in the electricity storage device at the previous night.